Modern wind turbines are generally equipped with electric pitch systems that have at least one pitch drive. In most cases, at least one pitch drive is provided for each rotor blade. As the rotor blades rotate around their longitudinal axis, such pitch systems regulate the position of the rotor blades relative to the wind, and they are often the only reliable way to bring the rotor of a wind turbine to a standstill. This is done in that the pitch drive or drives turn the rotor blades into the so-called feathering position, bringing the rotor to a standstill since it is no longer driven by the wind. The pitch system is normally supplied with power by the network into which the wind turbine also feeds the power it generates. If the network fails, a hazardous situation can arise, for example, if the rotational speed of the rotor of the wind turbine exceeds the permissible maximum value if the wind picks up, so that the wind turbine could sustain damage or persons present in the vicinity could suffer injury as a result.
In order to avert such a hazardous situation, even if the network fails, it must be possible to move the rotor blades into the feathering position, even when the pitch system is not being supplied with power by the external network. For this purpose, it is known from the state of the art to equip the pitch system with one or more emergency power supply devices that, in case of a network failure, ensure the supply of power to the pitch system and thus the functionality of the pitch system, at least until the rotor blades have been moved into the safe feathering position.
Pitch drives can have, for example, an inverter and an electric pitch motor. Here, the inverter is supplied with electric power by an input rectifier, whereby the inverter operates the pitch motor with this power. Direct-current motors as well as alternating-current motors are options as pitch motors.
Pitch systems known from the state of the art have the drawback that they are sensitive to overvoltages and that they can no longer remain in operation, even if the overvoltages in the supply network are small.
Overvoltages in the supply network can be caused, for instance, by lightning strikes. The voltages to which the pitch system that is to be protected is exposed vary greatly, depending on the point of strike of the lightning and on the way in which the lightning energy enters the supply network. Therefore, the state of the art employs overvoltage protection devices to protect the pitch system already against small overvoltages. Generally known overvoltage protection devices use, for example, varistors, suppressor diodes or gas discharge tubes. As a rule, varistors are used to protect pitch systems.
If, in spite of the presence of overvoltage protection devices, a pitch system is exposed to even just a brief overvoltage, an emergency procedure is immediately initiated for safety reasons, that is to say, all of the rotor blades are turned into the feathering position.
Before this backdrop, the objective of the invention is to put forward a method for the operation of a pitch system of a wind turbine that, even in case of moderate network overvoltages, permits the continued operation of the pitch system.